Even after the removal of compilation tools from the run image in #273 (which removed both ld and gcc), ldconfig still exists in the run image, so libraries in the base image can still successfully be found by ctypes.util.find_library.
As such, when only ldconfig is available (and not ld), find_library() only returns libraries from the base image, and not any installed via eg the APT or other buildpacks (which rely upon LD_LIBRARY_PATH since they have to do userland "installs" of libraries due to not having root permissions).
Therefore, we sadly have to include binutils (which is the package that provides ld) in the run image after all - though thankfully doing so only forgoes 19 MB of the original 203 MB run image size reduction from removing gcc, make and libc6-dev - so we're still well ahead.
Python's ctypes.util.find_library is used to find libraries at runtime.
find_library()calls out to various external programs to help with this task, initially tryingldconfig, and then falling back togccandldif needed - as documented here: https://docs.python.org/3/library/ctypes.html#finding-shared-librariesEven after the removal of compilation tools from the run image in #273 (which removed both
ldandgcc),ldconfigstill exists in the run image, so libraries in the base image can still successfully be found byctypes.util.find_library.However, whilst the
find_library()docs say that it supportsLD_LIBRARY_PATH, the docs fail to mention that the env var is only honoured whenldcan be found: https://github.com/python/cpython/blob/e870c852c0ea96fa4e4569e9c39c7ceb80ce858d/Lib/ctypes/util.py#L315-L320As such, when only
ldconfigis available (and notld),find_library()only returns libraries from the base image, and not any installed via eg the APT or other buildpacks (which rely uponLD_LIBRARY_PATHsince they have to do userland "installs" of libraries due to not having root permissions).One such popular use of
ctypes.util.find_libraryis in Django's GIS feature, which uses it to locate the GDAL library at runtime - which will stop working with buildpack-installed GDAL ifldis not in the run image: https://github.com/django/django/blob/4971a9afe5642569f3dcfcd3972ebb39e88dd457/django/contrib/gis/gdal/libgdal.py#L53-L66Therefore, we sadly have to include binutils (which is the package that provides
ld) in the run image after all - though thankfully doing so only forgoes 19 MB of the original 203 MB run image size reduction from removinggcc,makeandlibc6-dev- so we're still well ahead.GUS-W-15821115.